Just to add a small thing to what Sjaak (the excellent moderator of the Artlantis forums) said...
When you save an atl file, it references all materials/textures by paths on the local machine - perhaps scattered all over your disk and server. When you save an archive, it gathers all of these things together into a single folder structure so that you can move the project to another computer - whether it is another Mac or PC.
File paths on Windows and Mac OS X are entirely different, and so using an archive is the most reasonable approach to moving an atl file from one OS to the other.
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB